Proposal: Component Model v0

Currently, the component model is very complicated and difficult to understand and to implement. In particular, wasmtime has the only working implementation of the component nodel: wasmtime-environ, which is used by Rust dependents to lower the component model into a representation they can handle, which still has many layers of confusing abstraction.

I propose we create a minimal subset of the component model which can be implemented by other, external, runtimes and compilers, including:

• Components are not allowed to be nested, imported, or exported
• Async is disallowed, both in ABI options and types
• Encodings other than the native encoding, either UTF-8 or WTF-16, are disallowed
• externrefs are disallowed as handle types

This subset, which I propose to call Component Model v0, should include most single-module components, allowing external tools to parse this simplified representation and thus implement WASI 0.2 for it. At a later point, when the external tool is fully developed, the full component model might be implemented within that tool.

First mention of this complexity

TODO: compare with wasit2
TODO: finalize restrictions

Start by reviewing the proposed restrictions in this issue and the linked discussion in WASI issue 617. Compare the proposal with wasit2 and inspect how wasmtime-environ represents the component model. Done means the Component Model v0 restrictions are finalized and the minimal subset is clearly specified.